The present invention relates to electrodes for use in secondary electrochemical cells. More particularly, it concerns a negative electrode composition and matched positive electrode within a cell which exhibits an overcharge capacity at high cell voltage.
A substantial amount of work has been done in the development of high-temperature, secondary electrochemical cells. Positive electrodes for these cells have included chalcogens such as sulfur, oxygen, selenium or tellurium as well as their transition metal chalcogenides. Positive electrode materials such as the sulfides of iron, cobalt, nickel and copper are of current interest.
In high temperature cells, current flow between electrodes often is transmitted by molten electrolytic salt. Particularly useful salts include compositions of the alkali metal halides and/or the alkaline earth metal halides ordinarily incorporating a salt of the negative electrode reactant metal, e.g. lithium. In cells operating at moderate temperatures, aqueous and organic base electrolytes are permissable and these also can include cations of the negative electrode metal.
Alkali metals such as lithium, sodium, potassium or alkaline earth metals including calcium, magnesium, and others and alloys of these materials are contemplated as negative electrode reactants. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-magnesium, calcium-magnesium, calcium-aluminum, calcium-silicon and magnesium-aluminum have been investigated to maintain the negative electrode in solid form and thereby improve retention of the active material at high cell operating temperatures.
One of the disadvantages connected with a battery comprised of a plurality of series connected secondary electrochemical cells is the difficulty in equalizing the charge to the cells so that all cells are fully charged yet none is overcharged which may result in severe damage to various portions of the cell, such as to the current collectors. A method for equalizing a plurality of cells is disclosed in the Cox U.S. Pat. No. 4,079,303 issued Mar. 14, 1978 for Charging System And Method For Multicell Storage Batteries. This system involves a complex electronic equalizing apparatus which performs the equalization of the cells during charging.
It is desirable to design a secondary electrochemical cell which facilitates the equalization of a plurality of series connected cells without the expensive electrical method and apparatus disclosed in the Cox patent. Providing overcharge capacity to the cells permits equalization on the discharge side of the cell. That is, if a plurality of series connected cells each have overcharge capacity, then inherent variations in cell capacity, cell efficiency, cell temperature and other factors would not interfere with the equalization of the cells after a bulk charge. For example, parallel connection of a plurality of such cells would allow the cells having a higher charge to trickle charge cells having a lower charge.
An advantage of the lithium-aluminum electrode is the rise in the negative electrode potential (voltage discontinuity) at the end of the cell charge capacity which provides a ready indicator of when the cell is fully charged. This is an important feature of the lithium-aluminum electrode for preventing overcharge damage to the current collector and the like. One of the disadvantages of lithium alloy negative electrodes, such as lithium-aluminum electrodes has been the reduced cell voltage as compared to negative electrodes containing molten lithium. The reduced cell voltage and power have been accepted in order to obtain the enhanced electrode and cell stability afforded by solid lithium alloys.
Some of the disadvantages of lithium alloy negative electrodes and particularly the lithium-aluminum electrode have been avoided by electrodes disclosed in U.S. Pat. No. 4,158,720 issued June 19, 1979 to Kaun, one of the inventors herein, for Lithium-Aluminum Iron Electrode Compositions. Unfortunately, an electrode of the ternary alloy disclosed therein does not impart a second plateau in cell potential after the cell is fully charged, an advantageous aspect that can provide overcharge capacity to prevent overcharge damage to cell components.
It has been found that cells having a negative electrode which is a combination of a lithium-aluminum alloy and a ternary alloy including lithium and aluminum are endowed with an overcharge capacity which permits cell equalization on the discharge side while at the same time retaining the previously discussed very desirable characteristics of the lithium-aluminum electrode.
Literature pertinent to the subject matter of the present invention includes the Tomczuk et al U.S. Pat. No. 4,011,372, issued Mar. 8, 1977, for "Method of Preparing A Negative Electrode Including Lithium Alloy For Use Within A Secondary Electrochemical Cell". This patent discloses a particular method of electrode preparation and suggests the use of a lithium-aluminum, lithium-magnesium and lithium-silicon alloys. The reference, however, does not disclose or teach the use of lithium-aluminum ternary compositions as additives to lithium-aluminum electrodes to provide overcharge capacity thereto.
The Settle et al U.S. Pat. No. 3,957,532, issued May 17, 1976 for "Method of Preparing An Electrode Material of Lithium-Aluminum Alloy" discloses the various phases and compositions of lithium-aluminum alloys that are appropriate for use as a negative electrode material. However, no disclosure is made to suggest the addition of certain amounts of ternary lithium-aluminum alloys to provide electrode overcharge capacity.
The Buzzelli U.S. Pat. No. 3,607,413 issued Sept. 21, 1971 for "Method For Electrochemical Alloying of Aluminum and Lithium" teaches negative electrodes of lithium-aluminum alloys with the suggestion that they may contain less than 10 weight percent impurities of, for example, copper, magnesium manganese, indium and iron. No suggestion is made in this patent of a ternary lithium-aluminum alloy as a negative electrode additive to effect electrode overcharge capacity.